To the Writers and Director of “Hot Tub Time Machine”

Dave Goldberg, a physics professor at Drexel University and a noted time-travel expert, has written a letter to the writers and director of “Hot Tub Time Machine” with his criticisms. It begins:

Dear Sirs:

As a noted time-travel expert, I looked forward to your new film, “Hot Tub Time Machine” with great anticipation. Of course, one expects a fair amount of artistic license in movies. For example, even in 1985, the most energy efficient flux capacitors could be powered at far less than 1.21 Gigawatts of electricity. It is also wholly understandable that you have ignored (or are perhaps ignorant of) the vast literature on time machine design and have therefore built yours around a hot tub, which has been shown to be unstable, rather than around the more conventional wormhole. Further, you exhibit an admirable attention to detail on many particulars. Following on the Terminator model, you correctly realize that time travel may only be undertaken while in the nude.

However, I cannot stand idly by as you subject your characters to a number of inviolable paradoxes. Considered from least to most egregious: